The present disclosure relates to solid oxide fuel cells, and more particularly, to solid oxide fuel cells constituted by an integrated multi-layer structure type module.
Also, the present disclosure relates to a method of manufacturing the solid oxide fuel cells.
Fuel cells are devices that directly convert chemical energies of reactants such as fuel and oxidant into direct current (DC) electricity. Generally, various fuel cells such as a polymer electrolyte fuel cell (PEFC), a phosphoric acid fuel cell (PAFC), a molten carbonate fuel cell (MCFC), a solid oxide fuel cell (SOFC), and an enzymatic fuel cell are used in fuel cell fields.
Specifically, since the SOFC is operated at a high temperature of about 600° C. to about 1,000° C. to directly convert a hydrocarbon fuel into electricity, the SOFC has the highest energy conversion efficiency among fuel cells developed up to date. Typically, the SOFC includes a solid electrolyte having ionic conductivity with respect to oxygen ions and proton, a porous air electrode or an air electrode (or cathode) disposed on one side of the solid electrolyte, and a fuel electrode or an anode disposed on the other side of the solid electrolyte. Thus, oxygen gas or air containing oxygen is supplied into the air electrode, and a fuel gas containing hydrogen and hydrocarbon gas is supplied into the fuel electrode. As a result, a reduction reaction of oxygen occurs in the air electrode to generate oxygen ions. Then, the oxygen ions are moved into the fuel electrode through the solid electrolyte to generate water by reacting with the supplied hydrogen. Here, since electrons are generated in the fuel electrode and the electrons are consumed in the air electrode, electricity may be generated when the fuel electrode and the air electrode are connected to each other.
The SOFC may be largely divided into two kinds. One is a cylindrical type fuel cell in which an electrode and a solid electrolyte cover the circumference of a cylinder, and the other one is a flat plate type fuel cell in which a solid electrolyte and an electrode have flat shapes, respectively. Specifically, since the flat plate type fuel cell has a structure in which it is advantageous to increase a power density per unit volume through a high density thereof even though it is difficult to assemble the fuel cell, the flat plate type fuel cell may be advantageously used as a household or vehicle power source.
FIG. 1 is a schematic perspective view of a general flat type solid oxide fuel cell (SOFC) 1. Referring to FIG. 1, as described below in detail, the flat type solid oxide fuel cell 1 has a structure in which a plurality of connection members 4, each having a plurality of fuel paths 2 and air paths 3 on bottom and top surfaces thereof, is stacked. Also, an electrolyte layer 7 including a fuel electrode 5 on a top surface thereof and an air electrode 6 on a bottom surface thereof is inserted between the connection members 4. Also, a sealing member 8 is inserted into edges between the electrolyte layer 7 and the connection member 4.
In more detail, according to a general flat plate type SOFC 1, the plurality of fuel paths 2 through which fuel flows is disposed in the bottom surface of the connection member 4, and the plurality of air paths 3, which extend in a direction perpendicular to those of the fuel paths 2 to allow air to flow therethrough is disposed in the top surface. In addition, each of the fuel paths 2 and each of the air paths 3 have straight line groove shapes between wall parts 9, respectively. Also, when the plurality of connection members 4 is stacked, the electrolyte layer 7 including the fuel electrode 5 disposed on the top surface and the air electrode 6 disposed on the bottom surface is inserted between the connection members 4. Also, a sealing member 8 is inserted into the edges between the electrolyte layer 7 and the connection member 4. As described above, the electrolyte layer 7 including the fuel electrode 5 on the top surface that is one side thereof and the air electrode 6 on the bottom surface that is the other side thereof, the connection member 4 stacked so that the fuel paths 2 are disposed facing the fuel electrode 5 above the electrolyte layer 7, and the other connection member 4 stacked so that the air paths 3 are disposed facing the air electrode 6 below the electrolyte layer 7 are coupled to each other to constitute unit cells that are basic elements of the flat plate type SOGC 1. Actually, the plurality of unit cells that are the basic elements is stacked to complete the flat plate type SOFC 1.
Fuel gas and air are introduced through the fuel paths 2 and air paths 3 of each of the connection members 4, which are formed by each of the wall parts 9, to reach two electrode layers, i.e., the fuel electrode 5 and the air electrode 6. The fuel electrode 5 and the air electrode 6, which are the electrode layers have porous structures to easily cause an electrochemical reaction, respectively. Also, the electrolyte layer 7 has a dense structure to prevent the fuel gas and air from being ventilated with each other. Also, the sealing member 8 includes a sealing glass and seals the paths, which are respectively disposed on both surfaces of the connection material 4, to prevent the two kinds of gas from being mixed with each other. Such a flat plate type SOFC-related art is disclosed in Japanese Patent Laid-open Publication Nos. 2007-200710 and 2007-317594, U.S. Pat. Nos. 6,265,095, 6,183,897, and 4,997,726, and Europe Patent Laid-open Publication Nos. 2019443 and 993059.
In the flat plate type SOFC 1, the gases should be smoothly supplied into the two electrode layers, i.e., the fuel electrode 5 and the air electrode 6. Also, an insulation layer or plate formed of a material having sealing and insulating properties should be inserted into the remnant portion of the connection member 4, which does not contact the unit cells. However, since various components formed of materials different from each other are used to form a stacked structure of the related art SOFC, it may be difficult to secure reliability due to various factors such as a thermal expansion difference according to a high-temperature operation, oxidation, corrosion, and deterioration. Therefore, there is a limitation that the cells do not have a long life.